I imagine there are UI design best practices, like watching new users try out the site, that could be followed. A similarly serious approach I've seen is having a designated "help out the newbie" role, either as someone people are encouraged to approach or specifically pairing mentees with mentors.

Both of those probably cost more than they deliver. A more reasonable approach would be having two home pages: one for logged-in users that probably links to /r/all/new (or the list version), and one for new users that explains more about LW, and maybe has a flowchart about where to start reading based on interests.